1. Field of the Invention
The present invention relates to an electrophotographic apparatus in which contact charging is effected on an electrophotographic photosensitive member, and a process cartridge adapted for use in such apparatus.
2. Related Background Art
In the electrophotographic photosensitive member employed in the electrophotographic apparatus, inorganic photoconductive materials such as selenium, cadmium sulfide or zinc sulfide have principally been used as the photoconductive material, but organic photoconductive materials are widely used in recent years.
Organic dyes or pigments having a photoconductive property have the advantages of easier synthesis in comparison with the inorganic materials and freedom of selection of the compound showing photoconductivity in a suitable wavelength region.
The organic electrophotographic photosensitive members widely employed in the electrophotographic apparatus in recent years are of the function separation type in which the generation of charges and the transportation of charges are respectively conducted by different materials. The electrophotographic photosensitive members of such a function separated type can be divided into photosensitive members of a single-layer type in which a charge transporting material and a charge generating material are contained in the same layer, and photosensitive members of a laminated type including a charge generating layer containing the charge generating material and a charge transporting layer containing the charge transporting material, but the latter laminated type is commonly utilized. The electrophotographic photosensitive members of such laminated type are further divided into a type in which the charge generating layer is formed at the surface and a type in which the charge transporting layer is formed at the surface, but the latter structure is mainly utilized in order to protect the charge generating layer of a smaller thickness.
For the charging process in the electrophotographic apparatus employing such electrophotographic photosensitive member, the corona charging device has been widely employed. Such corona charging device, achieving charging by corona discharge in air, is very effective for uniformly charging the surface of the electrophotographic photosensitive member to a predetermined potential, but is also associated with certain drawbacks such as requiring a high-voltage power source and generating a large amount of ozone at the corona discharge.
In contrast, a contact charging device has the advantages that the power source can be of a lower voltage and that the generation of ozone is limited. The contact charging device charges the surface of the electrophotographic photosensitive member by applying a voltage to a charging member (such as a semi-conductive charging roller, a semi-conductive charging blade, a semi-conductive charging brush etc.) which is maintained in direct contact with the electrophotographic photosensitive member, and has recently been employed in place of the corona charging device, because of the above-mentioned advantages.
However, in an electrophotographic apparatus employing the laminated type electrophotographic photosensitive member having the charge transporting layer at the surface as in the prior art described above, the surface layer (charge transporting layer) of the photosensitive member is abraded and becomes thinner with the extended use of the electrophotographic apparatus, whereby the photosensitive member shows an increase in the electrical capacitance and a loss in the photosensitivity. As a result, the surface potential of the photosensitive member corresponding to a light area is not lowered sufficiently, whereby the potential contrast between the light potential and the dark potential becomes narrower. Therefore, for example in a normal developing system, if a sufficient developing contrast is desired in the image development, a sufficient inverse contrast cannot be given to the light potential, whereby the area corresponding to the light potential is developed with a low density to provide a so-called "fogged" image.
On the other hand, in the reversal developing system, the light potential is difficult to develop, whereby so-called a "low density" image is obtained.
The charging of the surface of the electrophotographic photosensitive member by the contact charging device gives rise to a significantly larger abrasion of the surface layer of the photosensitive member, in comparison with that provided by the corona charging device. Consequently the above-described "fogged" image appears earlier, and the service life of the electrophotographic photosensitive member or the electrophotographic apparatus employing such photosensitive member is shortened. Such a drawback becomes more conspicuous in case a pulsating voltage, obtained by superposing an AC voltage with a DC voltage, is applied to the charging means.